Template electrode structures for depositing active materials

ABSTRACT

Provided are examples of electrochemically active electrode materials, electrodes using such materials, and methods of manufacturing such electrodes. Electrochemically active electrode materials may include a high surface area template containing a metal silicide and a layer of high capacity active material deposited over the template. The template may serve as a mechanical support for the active material and/or an electrical conductor between the active material and, for example, a substrate. Due to the high surface area of the template, even a thin layer of the active material can provide sufficient active material loading and corresponding battery capacity. As such, a thickness of the layer may be maintained below the fracture threshold of the active material used and preserve its structural integrity during battery cycling.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/564,324, entitled “TEMPLATE ELECTRODE STRUCTURES FOR DEPOSITINGACTIVE MATERIALS,” filed Aug. 1, 2012, which is a divisional of U.S.patent application Ser. No. 13/039,031, entitled “TEMPLATE ELECTRODESTRUCTURES FOR DEPOSITING ACTIVE MATERIALS,” filed Mar. 2, 2011, whichclaims the benefit of priority to U.S. Provisional Application No.61/310,183, filed Mar. 3, 2010, entitled “ELECTROCHEMICALLY ACTIVESTRUCTURES CONTAINING SILICIDES,” and which is also acontinuation-in-part of U.S. patent application Ser. No. 12/437,529,entitled “ELECTRODE INCLUDING NANOSTRUCTURES FOR RECHARGEABLE CELLS”filed on May 7, 2009, all of which are incorporated herein by thisreference for all purposes.

STATEMENT OF GOVERNMENT SUPPORT

The invention described and claimed herein was made with United StatesGovernment support under NIST ATP Award No. 70NANB10H006, awarded by theNational Institute of Standards and Technology. The United StatesGovernment has certain rights in this invention.

BACKGROUND

The demand for high capacity rechargeable batteries is strong andgrowing stronger each year. Many applications, such as aerospace,medical devices, portable electronics, and automotive applications,require high gravimetric and/or volumetric capacity cells. Lithium ionelectrode technology provided some improvements in this area. However,to date, lithium ion cells are mainly fabricated with graphite, whichhas a theoretical capacity of only 372 mAh/g.

Silicon, germanium, tin, and many other materials are attractive activematerials because of their high electrochemical capacity. For example,silicon has a theoretical capacity of about 4200 mAh/g, whichcorresponds to the Li_(4.4)Si phase. Yet, many of these materials arenot widely used in commercial lithium ion batteries. One reason is thatsome of these materials exhibit substantial changes in volume duringcycling. For example, silicon swells by as much as 400% when charged toits theoretical capacity. Volume changes of this magnitude can causesubstantial stresses in the active material structures, resulting infractures and pulverization, loss of electrical and mechanicalconnections within the electrode, and capacity fading.

Conventional electrodes include polymer binders that are used to holdactive materials on the substrate. Most polymer binders are notsufficiently elastic to accommodate the large swelling of some highcapacity materials. As a result, active material particles tend toseparate from each other and the current collector. Overall, there is aneed for improved applications of high capacity active materials inbattery electrodes that minimize the drawbacks described above.

SUMMARY

Provided are examples of electrochemically active electrode materials,electrodes using such materials, and methods of manufacturing suchelectrodes. Electrochemically active electrode materials may include ahigh surface area template containing a metal silicide and a layer ofhigh capacity active material deposited over the template. The templatemay serve as a mechanical support for the active material and/or anelectrical conductor between the active material and, for example, asubstrate. Due to the high surface area of the template, even a thinlayer of the active material can provide sufficient active materialloading and corresponding electrode capacity per surface area. As such,the thickness of the active material layer may be maintained below itsfracture threshold to preserve its structural integrity during batterycycling. The thickness and/or composition of the active layer may alsobe specifically profiled to reduce swelling near the substrate interfaceand preserve this interface connection.

In certain embodiments, an electrochemically active electrode materialfor use in a lithium ion cell includes a nanostructured templatecontaining a metal silicide and a layer of an electrochemically activematerial that coats the nanostructured template. The electrochemicallyactive material is configured to take in and release lithium ions duringcycling of the lithium ion cell. Further, the nanostructured templatemay facilitate the conduction of electrical current to and from theelectrochemically active material. An electrochemically active electrodematerial may also include a shell formed over the layer of theelectrochemically active material. The shell may include carbon, copper,a polymer, a sulfide, and/or a metal oxide.

Examples of a metal silicide in the nanostructured template includenickel silicide, cobalt silicide, copper silicide, silver silicide,chromium silicide, titanium silicide, aluminum silicide, zinc silicide,and iron silicide. In a specific embodiment, a metal silicide includesat least one different nickel silicide phase among Ni₂Si, NiSi, andNiSi₂. An electrochemically active material may be crystalline silicon,amorphous silicon, a silicon oxide, a silicon oxy-nitride, a tincontaining material, a germanium containing material, and a carboncontaining material. An electrochemically active material may have atheoretical lithiation capacity of at least about 500 mAh/g or, morespecifically, of at least about 1000 mAh/g. Active materials with suchcapacities may be referred to as “high capacity active materials.” Incertain embodiments, an electrochemically active electrode material maybe used for fabricating a positive electrode. Examples of positiveelectrochemically active materials include various active components inthe form of LiMO₂, M representing one or more ions with an averageoxidation state of three. Examples of these ions include vanadium (V),manganese (Mn), iron (Fe), cobalt (Co), and nickel (Ni). The inactivecomponent may be in the form of Li₂M′O₃, M′ representing one or moreions with an average oxidation state of four. Examples of these ionsinclude manganese (Mn), titanium (Ti), zirconium (Zr), ruthenium (Ru),rhenium (Re), and platinum (Pt). Other positive active materials includesulfur, lithium iron silicates (Li2FeSiO4), hexavalent iron sodiumoxides (Na2FeO4).

In certain embodiments, a layer of the electrochemically active materialis doped to increase conductivity of the active materials. Some examplesof dopants include phosphorous and/or boron. In certain embodiments, ananostructured template includes silicide containing nanowires. Thenanowires may be between about 1 micrometers and 200 micrometers inlength on average and/or less than about 100 nanometers in diameter onaverage. A layer of the electrochemically active material is at leastabout 20 nanometers in thickness on average. In these or otherembodiments, a mass ratio of the active material to the template is atleast about 5.

In particular embodiments, a layer of the electrochemically activematerial includes amorphous silicon. This layer may be at least about 20nanometers thick on average. Further, a nanostructured template includesnickel silicide nanowires that are between about 1 micrometers and 200micrometers in length on average and less than about 100 nanometers indiameter on average.

Also provided is a lithium ion electrode for use in a lithium ion cell.In certain embodiments, a lithium ion cell electrode includes anelectrochemically active electrode material containing a nanostructuredtemplate and a layer of an electrochemically active material coating thenanostructured template. The nanostructured template may include a metalsilicide. The template may facilitate conduction of electrical currentto and from the electrochemically active material. The electrochemicallyactive material may be configured to take in and release lithium ionsduring cycling of the lithium ion cell. The electrode may also include acurrent collector substrate in electrical communication with theelectrochemically active electrode material. The substrate may include ametal of the metal silicide.

In certain embodiments, a nanostructured template of the electrodeincludes nanowires rooted to the substrate. In some cases, the ratio ofthe surface area of the nanostructured template to the surface area ofthe substrate is at least about 20. The substrate may include a basesub-layer substantially free of the metal of the metal silicide and atop sub-layer containing the metal of the metal silicide. The substratemay include copper, nickel, titanium, and/or stainless steel. Asubstrate for positive electrodes may also include aluminum.

The electrochemically active electrode material may include multiplestructures having free ends and substrate-rooted ends. Each of thesemultiple structures includes a nanostructured template andelectrochemically active material. In certain embodiments, theelectrochemically active material coats (at least partially) thetemplates. The active material layer may have a varying thickness and/orcomposition along the height of the template (e.g., along the length ofa nanowire template). In a specific embodiment, the active material isat least twice as thick at the free ends of the structures than at thesubstrate-rooted ends. In the same or other embodiments, theelectrochemically active material includes amorphous silicon andgermanium. The material may have more silicon and less germanium at thefree ends of the structures than at the substrate-rooted ends.

Also provided is a method of fabricating a lithium ion cell electrodefor use in a lithium ion cell. In certain embodiments, a method includesreceiving a substrate, forming a nanostructured template containing ametal silicide on a surface of the substrate, and forming a layer of anelectrochemically active material on the nanostructured template. Theelectrochemically active material is configured to take in and releaselithium ions during cycling of the lithium ion cell. The nanostructuredtemplate is configured to facilitate conduction of electrical current toand from the electrochemically active material. Furthermore, thetemplate provides structural support to the electrochemically activematerial as further described below.

In certain embodiments, a method also includes treating the substrateprior to forming the metal silicide template. This treatment may involveone or more of the following techniques: oxidation, annealing,reduction, roughening, sputtering, etching, electroplating,reverse-electroplating, chemical vapor deposition, nitride formation,and depositing an intermediate sub-layer. A method may also includeforming a metal component on the surface of the substrate such that aportion of the metal component is consumed when forming the metalsilicide.

In certain embodiments, forming the nanostructured template includesflowing a silicon containing precursor over the surface of thesubstrate. A method may also include doping the electrochemically activematerials. A method may also include forming a shell over the layer ofthe electrochemically active material. The shell may include one or moreof the following materials: carbon, copper, a polymer, a sulfide, afluoride, and a metal oxide.

In certain embodiments, the method also involves selectively depositinga passivation material over the nanostructured template prior to formingthe layer of the electrochemically active material. The passivationmaterial may include individual structures forming a layer and havingdiscrete spacing in between these structures.

In certain embodiments, forming the layer of the electrochemicallyactive material is performed in a mass transport regime such that asubstantially lower concentration of an active material precursor isavailable at the surface of the substrate than at free ends of thenanostructured template. The method may also involve changing thecomposition of active material precursors while forming the layer of theelectrochemically active material. This would allow production of, forexample, the graded germanium/silicon nanostructures described above.

These and other features will be further described below with referenceto the specific drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a process example of fabricating an electrochemicallyactive material containing a metal silicide template and a high capacityactive material.

FIG. 2A is a schematic representation of a three-layered substrateexample.

FIG. 2B is a schematic representation of clustered silicide structurescoated with the active material layer that overlaps near the bases ofthe silicide structures, forming bulky active material agglomerates.

FIG. 2C is a schematic representation of separated silicide structuresformed through a masking intermediate sub-layer, in accordance withcertain embodiments.

FIG. 2D is a schematic representation of the separated silicidestructures coated with the active material layer that does not overlapnear the bases of the silicide structures.

FIGS. 2E and 2F are schematic representations of uncoated silicidestructures with a deposited passivation material and coated silicidestructures where the passivation material prevented deposition of theactive material near the bases of the silicide structures.

FIG. 3A illustrates an example of initial, intermediate, and finalelectrode structures that may be present at different stages of thefabrication process described in the context of FIG. 1.

FIG. 3B illustrates an example of an electrode structure with unevendistribution of the high capacity active material.

FIG. 4A is a top down scanning electron microscope (SEM) image of thenickel silicide nanowires forming a high surface area template over thenickel coating.

FIG. 4B is a top down SEM image of the amorphous silicon deposited overthe nickel silicide nanowires similar to the ones shown in FIG. 4A.

FIG. 4C is a side SEM image of the electrode active layer containing thenickel silicide nanowires coated with the amorphous silicon.

FIG. 4D is a high magnification SEM image similar to the one presentedin FIG. 4B.

FIG. 4E is an SEM image obtained at an angle with respect to the topsurface of the electrode and illustrating nanowires being much thickerat their free ends than at their substrate-rooted ends.

FIGS. 5A-B are a top schematic view and a side schematic view of anillustrative electrode arrangement, in accordance with certainembodiments.

FIGS. 6A-B are a top schematic view and a perspective schematic view ofan illustrative round wound cell, in accordance with certainembodiments.

FIG. 7 is a top schematic view of an illustrative prismatic wound cell,in accordance with certain embodiments.

FIGS. 8A-B are a top schematic view and a perspective schematic view ofan illustrative stack of electrodes and separator sheets, in accordancewith certain embodiments.

FIG. 9 is a schematic cross-section view of an example of a wound cell,in accordance with embodiments.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Nanostructures, and in particular nanowires, are exciting new materialsfor battery applications. It has been proposed that high capacityelectrode active materials can be deployed as nanostructures and usedwithout sacrificing battery performance. Even major swelling duringlithiation, such as is observed with silicon, does not deteriorate thestructural integrity of nanomaterials because of their small size.Stated another way, nanostructures possess a high surface area to volumeratio in comparison to conventional electrode morphologies.Additionally, the high surface area to volume ratio provides a greaterfraction of the active material is directly accessible toelectrochemically active ions from the electrolyte.

Various embodiments are described herein with reference to nanowires. Itshould be understood, however, that unless otherwise stated, thereferences herein to nanowires are intended to include other types ofnanostructures including nanotubes, nanoparticles, nanospheres,nanorods, nanowhiskers, and the like. Generally, the term“nanostructures” refers to structures having at least one dimension thatis less than about 1 micrometer. This dimension could be, for example, adiameter of the nanostructure (e.g., a silicide template nanowire), athickness of the shell formed over a template (e.g., a thickness of theamorphous silicon layer), or some other nanostructure dimension. Itshould be understood that any of the overall dimensions (length anddiameter) of the final coated structure do not have to be at ananoscale. For example, a final structure may include a nano-layer thatis about 500 nanometers in thickness and coated over a template that isabout 100 nanometers in diameter and 20 micrometers in length. Whilethis overall structure is about 1.1 micrometers in diameter and 20micrometers in length, it could be generally referred to as a“nanostructure” because of the dimensions of the template and activematerial layer. In specific embodiments, the term “nanowire” refers tostructures with nano-scaled shells positioned over elongated templatestructures.

Nanowires (as a specific case of nanostructures) have an aspect ratio ofgreater than one, typically at least about two and more frequently atleast about four. In specific embodiments, nanowires have an aspectratio of at least about 10 and even at least about 100. Nanowires maymake use of their one larger dimension to connect to other electrodecomponents (e.g., a conductive substrate, other active materialstructures, or conductive additives). For example, nanowires may besubstrate rooted such that one end (or some other part) of the majorityof the nanowires is in contact with the substrate. Because the two otherdimensions are small and there is an adjacent void volume available forexpansion, the internal stress built up in the nanowires duringlithiation (e.g., expansion of the nano-shells positioned over thesilicide templates) is also small and does not break apart the nanowires(as happens with larger structures). In other words, certain dimensionsof the nanowires (e.g., an overall diameter and/or a shell thickness)are kept below the corresponding fracture levels of the active materialused. Nanowires also permit a relatively high capacity per unit area ofthe electrode surface due to their elongated structure, whichcorresponds to the height of the template structure. This results fromtheir relatively high aspect ratio and terminal connection to thesubstrate.

Depositing nanostructures containing high capacity materials may be aslow process that requires expensive materials, such as the goldcatalyst used in a Vapor-Liquid-Solid (VLS) deposition process. Batteryelectrodes produced using such processes may be cost prohibitive forcertain consumer applications, such as portable electronics andelectrical vehicles. Furthermore, VLS deposition typically yieldscrystalline structures, which are more rigid than amorphous structuresand, therefore, more susceptible to cracking and pulverization. Finally,a substrate connection of the VLS-deposited structures may be weak dueto the distinct interface of two different materials (e.g., metallicsubstrate and high capacity active material), one of which undergoessubstantial swelling while the other one remains intact. Without beingrestricted to any particular theory, it is believed that these phenomenacould undermine the cycling performance of the batteries built from suchelectrodes.

It has been found that some metal silicide nanostructures can be formeddirectly on certain substrates without using catalysts. The silicidestructures may be formed on surfaces that contain the metal making upthe metal silicide. The metal containing substrate surfaces may beprovided in various forms, such as a base sub-layer (e.g., a foil) or aseparate sub-layer positioned over a base current collector (e.g., athin nickel layer formed on a surface of a stainless steel or copperfoil). In some examples, the metal containing surfaces are treated priorto the formation of silicide structures in order to promote the silicideformation process. For example, a surface having a nickel-containingsurface may be oxidized prior to forming nickel silicide nanostructures.As further explained below, this oxidation creates nucleation points fornickel silicide formation. Overall, it has been found that oxidationallows a broader processing window during the template formation.

Silicide nanostructures can serve as a high surface area template thatis later coated with high capacity active materials forming a“composite” electrode. For purposes of this document, a “template”generally includes a collection of nanostructures used for supportingactive materials in the battery electrode. The template may provide bothmechanical support and/or electrical communication to the activematerial with respect to, for example, a conductive substrate. Incertain embodiments, the template is arranged as a layer adjacent to thesubstrate and may be characterized by its height or thickness. Such anarrangement may be referred to as a “template layer,” which should bedistinguished from other types of layers, such as an active materiallayer. This distinction is further pointed out in the description below.An adjacent substrate may be present in some but not all embodiments. Incertain embodiments, a template coated with an active material may bedirectly connected to other conductive elements of the cell (other thana conductive substrate), such as electrical lead wires and batteryterminals. In specific embodiments, a template may include a singlelayer of silicide nanowires extending generally away from the substrate,and in some embodiments in substantially the same direction. The heightof this template will generally correspond to the average length of thenanowires. However, it should be understood that other silicidestructural arrangements are also possible (e.g., multi-layered silicidetemplates).

A “template structure” generally refers to an individual structure thatis a part of the template. Some template structures include silicidematerials, while some structures in the same template may include othermaterials (e.g., conductive additives). Typically, template structureshave at least one nano-scaled dimension (e.g., a diameter). Therefore,such template structures may be referred to as template nanostructures.In some embodiments, the template nanostructures may be shaped asnanowires with substrate rooted ends (or other portions) that form anintegral structure with the substrate. In other words, they may not havea clearly defined morphological boundary or interface with the substratesurface to which the silicide nanowires are attached. As a result,substrate rooted nanowires may have superior mechanical adhesion to thesubstrate and low electrical contact resistance, for example, incomparison to the VLS-deposited structures Further, many silicides aregood electrical conductors and can provide a highly conductive pathbetween the active material deposited around the silicide nanowires and,for example, a current collecting substrate.

Metal silicides can also act as active materials themselves and besubjected to lithiation. However, silicides generally have far lowercapacity than, for example, silicon or tin. Therefore, a silicidetemplate may contribute comparatively less to the overall capacity ofthe electrode. This contribution may be particularly small when there issubstantially more active material than there is silicide material. Forexample, silicide nanowires that are only about 10 nanometers indiameter may be used to deposit an active layer that is at least about100 nanometers in thickness or, more specifically, between about 300nanometers and 500 nanometers in thickness. In this example, a ratio ofthe active material volume to the silicide volume is at least about 400.Therefore, such composite electrodes may be used with substantially nolithiation of the silicide template. Minimal or substantially nolithiation of the silicide structures helps to preserve their integrityas a template and the integrity of their connections to the substrate.These characteristics lead to strong and robust mechanical andelectrical connections within the electrode and, as a result, stablecycling performance over a large number of cycles. Various otherfeatures, such as cone-shaped silicide structures with thicker bases andcone-shaped (or mushroom-shaped) active material layers with thickerfree-ends, may be used to help maintaining these connections. Thesefeatures are typically focused on reducing swelling near the substrateinterface using various techniques.

A silicide template containing nanowires has a large surface areaavailable for supporting active materials. In certain embodiments,nanowires employed as the template are between about 10 nanometers and100 nanometers in diameter and between about 10 micrometers and 100micrometers in length. The nanowires may be densely spaced. Templatestructures that are closely spaced may share a common coating shelleffectively forming a multi-core single shell arrangement. In suchcases, the template growth density does not necessarily correspond tothe density of the coated nanostructures. In certain embodiments,spacing between template structures may be even less than the coatingthickness, thereby causing significant interconnections of the activematerial layer. These interconnections are particularly prominent nearthe bases creating agglomerated or continuous film like structures,which impede good cycle performance. Generally, it is desirable to avoidnanowires agglomerates, which are sometimes referred to as “bunches” or“bush-like” aggregates, further described with reference to FIG. 2B.

Often the template has a surface area that is orders of magnitudegreater than that of a typical substrate. The template can be coatedwith a thin layer of the active material and, thereby, provide anelectrode having a substantial reversible energy density. It should benoted that an active material layer does not necessarily require acontinuous layer extending over the entire template and, in someembodiments, over the substrate. In some embodiments, an active materiallayer is a collection of active material shells positioned over silicidestructures. Some of these shells may be disjoined at the substrateinterface, for example, by providing passivation materials at thesubstrate interface. Various examples of the active material layer aredescribed below. The thickness of the active material layer is generallydetermined by the characteristics of the active material used andgenerally kept below the fracture limit for the particular activematerial.

The thickness of the active layer coated over a template should bedistinguished from the thickness of the battery electrode. The thicknessof the active layer is generally nano-scaled, while the thickness of thebattery electrode generally corresponds to at least the height of thetemplate and could be tens of micrometers. It should be noted thattemplate structures (e.g., silicide nanowires) are typically notperfectly vertical. Therefore, the template height may be somewhat lessthan the lengths of these structures. Generally, the conductivesubstrate also contributes to the thickness of the electrode. In oneexample, a 100 nanometer thick silicon layer deposited over 10micrometer long nanowires that are 10 nanometers in diameter and spacedapart by 500 nanometers can provide an energy density comparable to thatof a conventional graphite negative electrode that is substantiallythicker. As such, electrochemical cells with improved gravimetric andvolumetric capacity characteristics can be constructed using theseactive material structures and electrodes.

Once the template is formed, active materials can be deposited as alayer over this template in a relatively fast manner and without a needfor expensive catalysts. Further, certain deposited active materials maytake some more desirable morphological forms. For example, acatalyst-free deposition over nickel silicide nanowires yields amorphoussilicon, while growing silicon nanowires from gold catalyst islandsusing VLS yields crystalline silicon. Without being restricted to anyparticular theory, it is believed that amorphous silicon structures havefewer and weaker atomic bonds, which allows such structures to retaintheir integrity better than the more rigid crystalline structures whenexposed to the stress encountered during repeatedlithiation/delithiation cycles. Also, deposition techniques used to forman active material layer may be specifically tuned to controldistribution of the active material along the template height (e.g.,depositing more active material near free-ends of the active materialstructures than near the bases) and to control other characteristics ofthe deposited materials, such as composition, porosity, and others.

Furthermore, various techniques have been proposed to protect theelectrical connection between nanowires and conductive substrate. In oneclass of techniques, the structure of the completed nanowires has a “topheavy” shape in which a nanowire's attachment region, the region wherethe nanowire approaches and contacts the substrate, is relativelythinner than the distal region of the nanowire. Generally, the distalregion will have substantially more active material than the attachmentregion. In another class of techniques, the spacing of the templatenanowires is controlled such that the individual wires are relativelyevenly spaced in their attachment to the substrate. In specificembodiments, a mechanism is employed to prevent the template nanowiresfrom bunching near to one another at their attachment regions. In yetanother class, certain “passivation” techniques and/or materials areemployed to minimize mechanical distortions and stresses at thesubstrate interface that are generally caused by swelling andcontraction of the active materials.

Some examples of top heavy shapes include shapes that have gradually andcontinuously increased cross-sectional dimensions (e.g., diameter) fromthe substrate rooted ends to the free ends (similar to the ones shown inFIG. 3B). In other embodiments, the cross-sectional dimensions mayincrease gradually but not continuously. Other examples include shapesthat increase their cross-sectional dimensions abruptly butcontinuously. Furthermore, other examples include shapes that increasetheir cross-sectional dimensions abruptly and not continuously. Theoverall shape profile may be driven by the thickness of the activematerial layer, cross-sectional dimensions of the template structures,or a combination of these two parameters. For example, a templatestructure may have a wider base than free end, while a distribution ofthe active material coating may be such that the overall electrodestructure has a wider free end than the base.

FIG. 1 illustrates a process 100 of fabricating an electrochemicallyactive electrode containing a metal silicide template and a highcapacity active material, in accordance with certain embodiment. Theprocess may start with receiving a substrate (operation 102). Asubstrate material may be provided as a roll, sheet, or any other formthat is fed into a process apparatus used in one or more of subsequentoperations. Typically, the substrate is made from a material that canserve as an electrode current collector, although this need not be thecase (as explained below). Examples of suitable apparatus includeChemical Vapor Deposition (CVD) apparatus (e.g., Thermal CVD or a PlasmaEnhanced CVD apparatus), Physical Vapor Deposition (PVD) apparatus, andother apparatus suitable for performing the operations described below.In certain embodiments, one or more operations of the described processare performed in a vertical deposition apparatus described in U.S.patent application Ser. No. 12/637,727 entitled “Apparatus forDeposition on Two Sides of the Web” filed on Dec. 14, 2009 to Mosso etal., which is incorporated by reference herein in its entirety forpurposes of describing vertical deposition apparatuses.

The substrate is typically a part of the electrode (e.g., a currentcollector substrate). However, it may also be used as a temporarycarrier that supports the template and active material duringfabrication, and/or a source of materials during electrode fabrication(e.g., a source of metal in a metal silicide deposition operation), andthen removed, while the template is electrically connected to thecurrent collector components of the battery. If a substrate becomes apart of the electrode, it may generally include a material suitable foruse in this electrode (from mechanical, electrical, and electrochemicalperspectives). Examples include continuous foil sheets, perforatedsheets, expanded metals, and foams.

In certain embodiments, the substrate includes a metal containingmaterial, which metal is consumed to form metal silicide nanostructures.Examples of suitable metal containing materials are provided below. Themetal containing material may be supported on a base substratesub-layer, which serves as a mechanical support for the template and theactive materials. Alternatively or in addition, the base substratesub-layer may serve as an electrical current conductor between thesilicide nanostructures (and, to a lesser extent, the active materials)and the battery electrical terminals.

Various intermediate sub-layers may be provided in between the basematerial and the metal source. For example, a sub-layer containingcopper and/or nickel may be deposited between the base and metal sourcesub-layers to improve metallurgical and electronic connections of thelater-formed template to the base sub-layer. In a specific embodiment, abase sub-layer containing a conductive material (e.g., stainless steel)is coated with a thin sub-layer of copper followed by a thickersub-layer of nickel (e.g., between about 10 nanometers and 3micrometers). The nickel sub-layer is then used to form a nickelsilicide template, while the copper sub-layer acts as an adhesion andconductive intermediary.

In certain embodiments, the same material serves as both the currentcollecting base material and the metal source for the silicide template.Examples of materials that may be used as both a base material and ametal source for the silicide include nickel, copper, and titanium, bothof which may be provided as foils, perforated sheets, expanded metals,foams, and the like. In other embodiments, the substrate contains twomaterials that form distinct sub-layers or other structures (e.g., acopper base foil coated with a thin nickel layer). In some cases, themetal source material exists as discrete droplets, particles, or regularpatterns distributed throughout the base material. Typically, though notnecessarily, the metal containing material used to form the silicide ispositioned on the base material surface so that it is directly exposedto the processing environment (e.g., a silicon containing precursor gas)during processing. Generally, distribution of the two materials withinthe same structure may be uniform (an alloy or compound in the extremecase) or non-uniform (e.g., a gradual distribution with more metalsource material concentrating near the surface).

Examples of base materials include copper, copper coated with metaloxides, stainless steel, titanium, aluminum, nickel, chromium, tungsten,metal nitrides, metal carbides, carbon, carbon fiber, graphite,graphene, carbon mesh, conductive polymers, or combinations of the above(including multi-layered structures). The base material may be formed asa foil, film, mesh, foam, laminate, wires, tubes, particles,multi-layered structure, or any other suitable configuration. In certainembodiments, a base material is a metallic foil with a thickness ofbetween about 1 micrometer and 50 micrometers or, more specifically,between about 5 micrometers and 30 micrometers.

Examples of metal containing source materials include nickel, cobalt,copper, silver, chromium, titanium, iron, zinc, aluminum, tin and theircombinations. Examples of some alloys include nickel/phosphorus,nickel/tungsten, nickel/chromium, nickel/cobalt, nickel/iron,nickel/titanium, and nickel/molybdenum. As mentioned, in certainembodiments, a metal containing source material forms a source sub-layeron the top of the base material. Such a source sub-layer may be at leastabout 10 nm thick or, more specifically, at least about 100 nm. Incertain embodiments, a source sub-layer may be up to about 3 micrometersthick. In other embodiments, a metal containing material forms particlesor some other discrete structures on the surface of the base material.These discrete structures may be provided in a thickness of at leastabout 10 nanometers thick or, more specifically, between about 10nanometers and 50 micrometers. In general, a substrate should have asufficient amount of the metal containing material near or on thesubstrate surface to form silicide nanostructures. For example, a20-nanometer thick nickel sub-layer deposited over a copper basesub-layer may be sufficient to produce a dense mat of nickel silicidenanowires that are 20 micrometers long.

In certain embodiments, a thin sub-layer of a masking material is formedusing a PVD or some other deposition technique. A thickness of thissub-layer may be between about 1 Angstroms and 15 Angstroms. It has beenfound that certain materials at such thicknesses do not form acontinuous layer but instead form a collection of small separatedislands or clumps. Specifically, masking materials may be deposited assmall islands and used for masking the underlying substrate fromdepositing a metal containing sub-layer in these areas. Alternatively orin addition to, masking materials may be deposited on top of a metalcontaining sub-layer to mask template growth

In certain embodiments, a metal containing sub-layer may be patternedduring deposition of this sub-layer. For example, a masking sub-layer(e.g., a mesh) may be positioned over the base sub-layer and the metalcontaining sub-layer is formed over this combination. The coveredportions of the base sub-layer will be substantially free from the metaland will not form silicide structures during later operations. A testwas conducted using a metal mesh positioned over a substrate surface.Titanium was then deposited through the open spaces in the mesh, formingtitanium islands. These islands in turn blocked silicide formation inthese areas, which resulted in a patterned template growth. A specialmesh with small pitch may be fabricated using, for example, nano-imprintlithography or some self assembled techniques to achieved desireddistribution of the masking particles.

A substrate may contain other materials that may be used to enhance theadhesion of subsequently formed silicide nanostructures to the basesub-layer, to protect the base sub-layer during processing and cellcycling, to promote nucleation of the template structures, to preventdeposition of the active materials at (or near) the substrate interface,to act as an additional source of silicon during silicide formation, andother functions. For example, a substrate may include an intermediatesub-layer to perform such function. FIG. 2A is a schematicrepresentation of a three-layered substrate 200, in accordance withcertain embodiments. Sub-layer 202 is a base sub-layer, sub-layer 206 isa metal containing material sub-layer, and sub-layer 204 is anintermediate sub-layer. In certain embodiments (not shown), anintermediate sub-layer may be positioned on the other side of the metalcontaining sub-layer with respect to the base sub-layer (or thesubstrate). Additional examples and details of intermediate sub-layersare provided in U.S. Provisional Patent Application 61/260,297 toDelHagen et al., entitled “INTERMEDIATE LAYERS FOR ELECTRODEFABRICATION” filed on Nov. 11, 2009, which is incorporated herein byreference in its entirety for purposes of describing intermediatesub-layers. Still other materials and sub-layer can be provided as apart of substrate. For example, a metal containing sub-layer may have ametal oxide sub-layer or a protective sub-layer.

Returning to FIG. 1, a substrate received in operation 102 may have amasking sub-layer, which is positioned over the metal containingsub-layer. The masking sub-layer covers a portion of the metalcontaining sub-layer, while exposing certain small spaced-apart areas ofthe metal containing area. During formation of silicide structures inoperation 106, the exposed areas are more available to react withsilicon-containing precursors (e.g., silane), thereby resulting in theformation of discrete silicide structures such as the ones shown in FIG.2C as opposed to the silicide structure clusters shown in FIG. 2B.Specifically, FIG. 2B is a schematic representation of clusteredsilicide structures 214 coated with the active material layer 216 thatoverlaps near the bases of the silicide structures (i.e., near thesubstrate 212) and forms bulky active material agglomerates. The overalldimension of these agglomerates (or the thickness of the active materialnear the substrate interface) may greatly exceed threshold limits for aparticular active material, resulting in fractures and high stress nearthe interface during battery cycling. Not only may the active materialdelaminate from the silicide structures, but the entire silicidestructure may separate from the substrate, thereby making theminoperative.

Depositing a masking sub-layer may help to overcome such clustering.FIG. 2C is a schematic representation of separated silicide structures224 formed through a masking intermediate sub-layer 225 positioned overthe substrate 222, in accordance with certain embodiments. The maskingintermediate sub-layer 225 may have openings that determine where thesilicide structures 224 are formed, which allows for separating anddistributing silicide structures 224 based on templates defined by themasking intermediate sub-layer 225. The distribution of the templatestructures could be random or patterned. Examples of the maskingsub-layers include self-assembling zinc oxide particles and siliconoxide particles, and randomly oriented nanowires forming amesh-structure over the metal containing sub-layer. Some correspondingtechniques to form islands from a masking sub-layer or a metalcontaining sub-layer include evaporation, angle deposition,self-assembly, lithography patterning, and others.

FIG. 2D is a schematic representation of the separated silicidestructures 224 (similar to the ones depicted in FIG. 2C and describedabove) coated with an active material layer 226. The active materiallayer 226 does not overlap near the bases of the silicide structures 224to form agglomerates. As such, even at the substrate interface, theactive material layer 226 is within the fracture threshold, whichresults in less mechanical stress and pulverization than, for example,the structures deposited in FIG. 2B.

Masking sub-layers may remain as a part of the electrode or may beremoved. The masking sub-layer used to pattern the metal containingsub-layer may be mechanically removed prior to formation of the silicidestructures. The masking sub-layer used to cover portions of the metalcontaining sub-layer during formation of the silicide structures may bechemical removed (e.g., by selective etching of the masking sub-layerwithout substantially disturbing the silicide structures). Specificexamples include acid etching, heating, and evaporating. In otherembodiments, the masking sub-layer remains as a part of the electrodeand may be used, for example, to prevent deposition of the activematerial at the substrate interface. Some of these examples are furtherdescribed below with reference to FIGS. 2E and 2F.

It should be noted that substrate materials may interweave with eachother (e.g., particles of the metal containing sub-layer positionedamong particles of the intermediate sub-layer in a weave, felt, mesh, orcomparable structure). Further, it should be noted that distinctmaterials may be provided together as a part of the substrate introducedto the process in operation 102, or one or more such materials may bedeposited or otherwise integrated with the substrate in later processingoperations.

Returning to FIG. 1, the process 100 may proceed with an optionaltreatment of the substrate surface (operation 104). The treatment may beused to modify the substrate surface in order to enhance silicideformation or for other purposes. Examples of such treatment includeintroducing materials used in metal silicide formation (e.g., sources ofsilicon, sources of the metal, catalysts, and the like), chemicallymodifying the substrate surface (e.g., forming oxides, nitrides,carbides, initial silicide structures, and treatments with variousoxidizing and reducing agents), physically modifying the surface (e.g.,increasing surface roughness by laser ablation, knurling,electro-polishing (such as electroplating and reverse-electroplating toincrease the surface roughness), changing grain orientation, annealing,treating with oxygen based plasma to form an oxide, treating with argonbased plasma to change roughness (e.g., sputter cone formation),sonication, and ion implantation. It should be noted that some of thesetechniques may be used to control amounts of various materials (e.g., ametal source material) present on the surface as well as the physicalcharacteristics of these materials (e.g., surface roughness). Forexample, chemically modifying the substrate surface with reducing oroxidizing agents can be used to modify the roughness at a scaleparticularly useful for facilitating nucleation. Sonication in acetonefollowed by methanol and isopropanol rinses may be used o clean metalfoils prior to etching. Other techniques include oxygen plasma etching.Further, one may treat the surface with a dopant to increase theconductivity of the silicide structure if the dopant diffuses into thesilicon reacting metal.

In certain embodiments, a substrate containing a nickel coating or othersilicide source material on its surface is first oxidized. As mentionedabove, a bulk of the substrate may be made from a silicide sourcematerial. A specific example includes nickel foil. When a nickelsub-layer is used on a top of another substrate, the thickness of thenickel coating may be between about 50 nanometers and 300 nanometers forthe process conditions presented below. A temperature of the substrateduring oxidation/treatment may be maintained at between about 150° C.and 500° C. for between about 0.1 and 10 minutes in the presence ofoxygen or other suitable oxidant. In more specific embodiments, theoxidation is performed in the presence of air in a chamber maintained atabout 50 Torr for about one minute, while the substrate is kept at about300° C. The oxidation/treatment may proceed for between about 1-2minutes. In certain embodiments, no specific oxidation/treatmentoperation is present, and the process proceeds directly with formationof template structures. It is believed that residual moisture and oxygenpresent in a deposition chamber provide sufficient treatment of thenickel surface during process initiation and deposition stages. However,in order to achieve a more controlled formation of silicide template, aspecifically controlled oxidation operation may be needed. Specifically,it has been found that some oxidation helps formation of nickel silicidestructures. Without being restricted to any particular theory, it isbelieved that during oxidation, a smooth nickel surface converts to arougher nickel oxide surface. Rough oxide edges may serve as nucleationsites during later silicide formation. Further, the oxide may act as amask to allow nucleation only at the pores of the nickel coating. Thishelps to achieve a more even distribution of silicide nanowires andavoids clustering (as described above).

Another function of an oxide may be to regulate the diffusion rate ofthe metal from the source material sub-layer and to the reaction site.It has been found that excessive oxidation may be detrimental tosilicide formation. For example, when a flow of dry air of about 200sccm is mixed with argon at about 1-5% and used for oxidation at 400° C.for about 30 seconds, a resulting surface is believed to be excessivelyoxidized. Instead of forming a rough surface with multiple nucleationsites, a resulting over-oxidized surface has a golden color and causesnucleation of very few silicide nanowires. In the same manner, aninsufficiently oxidized surface may not provide sufficient nucleationsites. As such, oxidation conditions may be optimized for each metalcontaining material and the structures containing these materials.

The process 100 may proceed with the formation of silicidenanostructures (block 106). In certain embodiments, a substrate isintroduced into a CVD chamber. It should be noted that other operations,such as treatment operation 104 and/or active material formationoperation 108, may be performed in the same chamber. A siliconcontaining precursor, such as silane, is then flown into the chamber ata flow rate of, for example, between about 10 sccm and 300 sccm. Theseflow rate values are provided for the STS MESC Multiplex CVD systemavailable from Surface Technology Systems in United Kingdom, which canprocess substrates up to about 4 inches in diameter. However, one havingordinary skills in the art would understand that other CVD systems maybe used. The volumetric concentration of silane in the carrier gas maybe less than about 10% or, more specifically, less than about 5%, oreven less than about 1%. In particular embodiments, the concentration ofsilane is about 1%. A process gas may also include one or more carriergases, such as argon, nitrogen, helium, hydrogen, oxygen (althoughtypically not with silane), carbon dioxide, and methane. During silicidedeposition, the substrate may be maintained at a temperature of betweenabout 350° C. and 500° C. or, more specifically, between about 385° C.and 450° C. The chamber pressure may be between about 0.1 Torr andatmosphere pressure or, more specifically, between about 50 Torr and 300Torr. The duration of deposition may be between about 1 minute and 60minutes or, more specifically, between about 5 minutes and 15 minutes.

In certain embodiments, process conditions may be varied during the samedeposition cycle. For example, silane may be introduced initially at arelatively high concentration in order to promote the nucleation ofsilicide nanostructures. The silane concentration may be then reduced(e.g., towards the end of the silicide deposition operation) whenfurther nanowire growth is limited by metal diffusion from the rootedends of the nanowires towards the growing tips. Further, the substratetemperature may initially be kept low and then increased in order topromote such metal diffusion. Overall, process conditions may be variedto control physical properties, e.g., length, diameter, shape,orientation of template structures. Furthermore, morphologicalproperties of template structures, such as stoichiometric phases,crystalline/amorphous phases, and distribution of materials along theheight of the template, may be controlled by varying process conditions.Other process conditions to be considered are a composition of the gasmixture, flow rates, flow patterns, a chamber pressure, a substratetemperature, and electric field characteristics. In certain embodiments,process conditions (e.g., temperature, pressure, and silaneconcentration) are adjusted to promote sidewall deposition of amorphoussilicon or deposition of silicon particles onto the silicide structuresonce they have nucleated. Conditions that could be changed may includeprocess temperature, pressure, and silane concentration.

The chosen process conditions generally depend on a metal containingmaterial as well as size, morphology, and composition of desiredstructures. For example, the deposition conditions described above canbe used to grow nickel silicide nanowires that, on average, are betweenabout 0.5 micrometers and 50 micrometers in length and between about 10nanometers and 100 nanometers in diameter. A nickel coating that is atleast about 20 nanometers thick may be sufficient to deposit such nickelsilicide structures.

In general, silicide nanowires may between about 5 nanometers and 100nanometers in diameter (i.e., prior to depositing active material) or,more specifically, between about 10 nanometers and 50 nanometers.Further, nanowires may be between about 1 micrometer and 100 micrometerslong or, more specifically, between about 5 micrometers and 50micrometers long and even between about 12 micrometers and 30micrometers. Without being restricted to any particular theory, it isbelieved that silicide nanowire length may be limited by the diffusionof metal from the substrate to the growing tip. It has been found thatnickel silicide nanowires rarely grow longer than about 20 to 25micrometers when the process conditions described above are used.

While such length may provide an adequate surface area for activematerial deposition, certain techniques may be used to further elongatenanowires. In certain embodiments, an intermediate sub-layer with asilicon containing material is introduced between the base sub-layer andthe metal containing sub-layer. A silicon intermediate sub-layer canprovide an alternate (or additional) source of silicon in closerproximity to the root of growing nanostructures, which may aid thenucleation process. It has been found that silicide structures grownfrom nickel deposited on a silicon wafer nucleates much more uniformlyand grows more rapidly. In certain embodiments, an intermediatesub-layer includes a metal dopant that diffuses when silicon reacts withmetal and also increases the conductivity of the resulting silicide. Thedopant can be deposited or even implanted, particularly if provided in arelatively low quantity. In some cases, nitrogen is used to dope nickelsilicide.

In another embodiment, after forming an initial silicide template, anadditional metal containing material may be introduced (e.g., sputteredonto the initial template), and silicide formation operation 106 isrepeated. In other words, the initial silicide template becomes a newsubstrate for another silicide template that is deposited over it and soon. In this example, depositing another template may provide additionalcross-linking in the initial templates, thereby helping with themechanical and electrical integrity. Additional examples and details oftemplates and electrodes are provided in U.S. Provisional PatentApplication 61/347,614, entitled “MULTIDIMENSIONAL ELECTROCHEMICALLYACTIVE STRUCTURES FOR BATTERY ELECTRODE,” filed on May 24, 2010, andU.S. Provisional Patent Application 61/406,047, entitled “BATTERYELECTRODE STRUCTURES FOR HIGH MASS LOADINGS OF HIGH CAPACITY ACTIVEMATERIALS,” filed on Oct. 22, 2010, both of which are incorporatedherein by reference in their entirety for purposes of describingtemplates and electrodes.

Silicide nanowires are typically substrate rooted by virtue of growingfrom a metal containing material provide on the substrate. Certaindetails of substrate rooted structures are described in U.S. patentapplication Ser. No. 12/437,529 entitled “ELECTRODE INCLUDINGNANOSTRUCTURES FOR RECHARGEABLE CELLS” filed on May 7, 2009, which isincorporated herein by reference in its entirety for purposes ofdescribing substrate rooted structures. However, unlike some VLS grownnanowires described in that patent application, silicide nanowires mayform stronger mechanical bonds with the substrate and have lower contactresistance. It is believed that a variable material composition andwider substrate rooted ends contribute to this phenomenon.

It was found that silicide nanowires, when fabricated as describedherein, generally have a variable material composition along the lengthof the nanowire. Nanowires have a higher concentration of metal near thesubstrate rooted ends, where more metal is available, than near the free(distal) ends. Depending on the metal type, this variability may reflectin different morphological and stoichiometric phases of silicides. Forexample, a nickel silicide nanowire may include one, two, or all threephases of nickel silicide (i.e., Ni₂Si, NiSi, and NiSi₂). It is believedthat higher nickel content phases form stronger bonds with nickel metal.Therefore, this variability may strengthen the nickel silicide nanowiresadhesion to the substrate and reduce the contact resistance. Metalcontent variability may also cause different physical properties alongthe nanowires' length.

In particular embodiments, substrate rooted ends with the higher nickelcontent are wider and have higher surface roughness. This provides agreater contact area with the substrate, improves adhesion, and reducescontact resistance. Strong bonds between the substrate and nanowireshelp to preserve this attachment, particularly during cell cycling whenthe active material deposited onto nanowires swells and contracts andmay push the nanowires in various directions. Finally, in certainembodiments, silicide nanowires do not experience lithiation duringcycling.

Cone shaped nanowires, as described above, may result from a greateravailability of metal near the substrate rooted ends of the nanowires.In certain embodiments, an average diameter near the substrate rootedends is at least about twice that of an average diameter near the freeend (based on a comparison of the two sections at each end of thenanowire, with each section taken at a distance from the nanowire endthat is about 10% of the total nanowire length). In other words, basesmay be large enough to even touch each other on the surface of thesubstrate, but the tips are free and apart as a result of a decrease indiameter along the structure from the base to the tip. In more specificembodiments, a ratio of the two diameters is at least about 4 or, evenmore specifically, at least about 10 (representing wider base cones).

Silicide nanowires may interconnect with other nanowires, for example,when one nanowire crosses its path with another nanowire during theirgrowth. Further, additional cross-linking may be provided afterdepositing silicide nanowires. For example, another template may bedeposited over the first one, as described above. A conductive additive(e.g., carbon black, metallic particles) may be introduced among thenanowires. Nanowires may be reshaped after deposition to form morecontact points among nanowires, for example, by compressing and/orannealing the silicide template. Finally, additional interconnectionsmay occur during deposition of the active material. For example, twoclosely spaced silicide nanowires may be coated with an active materialsuch that the active material layers formed on the adjacent nanowiresoverlap. In a specific embodiment, forming a template is performed in aprocess chamber maintained at a pressure of about 50 Torr. The processgas contains about 1% of silane. The substrate is kept at about 450° C.

It should be noted that while the references in this document aregenerally made to a template including nanowires, the template mayinclude other types of structures. Further, wire-based templates mayinclude wires that have an average diameter greater than 1 micrometer.Such templates may be used to deposit a layer of high capacity activematerial such that the layer itself has nano-scale dimensionsirrespective of the template dimensions. However, templates made fromnanostructures, such as nanowires, generally provide greater surfacearea available for deposition of the high capacity active material.

After formation of the template but before depositing the activematerial, the template may be additionally processed to mask certainareas of the template in order to prevent or minimize deposition of theactive material in these areas. As described above, mechanicaldistortions, such as active material swelling and contraction, should beminimized near the substrate interface to preserve mechanical andelectrical bonds between the silicide template and substrate. As such,deposition of the active material near the substrate interface isgenerally not desirable or, at least, less desirable. Some techniques toprofile the thickness and/or composition of the active material layerduring deposition are described below with reference to active materialformation operation 108. Further, additional materials may be depositedat the substrate interface after formation of the template. It should benoted that such materials may be deposited in addition, or instead of,intermediate sub-layers provided prior to formation of the template,which are described above. To distinguish the two materials, thematerial deposited after formation of the template is referred to as a“passivation material” because it may be used, in certain embodiments,to passivate the substrate interface and reduce formation of the activematerial at this interface.

FIG. 2E is a schematic representation of uncoated silicide structures234 with a deposited passivation material 235. The passivation material235 deposited near the substrate 232 coats the substrate rooted ends ofthe silicide structures 234 while the free ends of these structuresremain uncoated. The passivation material 235 may be deposited during aseparate operation or during initial stages of active materialdeposition. For example, self-assembling zinc oxide and silicon oxideparticles may be introduced into the template. The distribution of thepassivation material 235 within the template may be provided byelectrodeposition.

FIG. 2F is a schematic representation of silicide structures 234 coatedwith an active material 236 such that the passivation material 235prevented deposition of the active material 236 near the bases of thesilicide structures 234. As such, little or no mechanical distortion andstress are present at the substrate 232 during cycling of the electrode,and the connection between the silicide structures 234 and substrate 232tend to be more robust.

In certain embodiments, an intermediate sub-layer is deposited over aformed template structure but before deposition of the electrochemicallyactive material. This sub-layer is positioned at the template-activematerial interface. This intermediate sub-layer may include titanium,copper, iron, nickel, nickel titanium, chromium or other similarmaterials. Materials may be deposited using electroplating, sputtering,or evaporation techniques. Without being restricted to any particulartheory, it is believed that a presence of an intermediate sub-layer atthis interface increases metallurgical alloying with the active materialand better adhesion. Further, some of these materials may act asadhesion promoters and oxygen getters. Finally, alloys like nickeltitanium, copper-zinc-aluminum-nickel, and copper-aluminum-nickel may beused for their elastic properties to provide an interface between arelative dynamic active material layer (which swells and contractsduring cycling) and relative static template layer.

Returning to FIG. 1, the process 100 continues with formation of a highcapacity electrochemically active material over the metal silicidetemplate (operation 108). Examples of electrochemically active materialsinclude silicon containing materials (e.g., crystalline silicon,amorphous silicon, other silicides, silicon oxides, sub-oxides,oxy-nitrides), tin containing materials (e.g., tin, tin oxide),germanium, carbon containing materials, a variety of metal hydrides(e.g., MgH₂), silicides, phosphides, and nitrides. Other examplesinclude: carbon-silicon combinations (e.g., carbon-coated silicon,silicon-coated carbon, carbon doped with silicon, silicon doped withcarbon, and alloys including carbon and silicon), carbon-germaniumcombinations (e.g., carbon-coated germanium, germanium-coated carbon,carbon doped with germanium, and germanium doped with carbon), andcarbon-tin combinations (e.g., carbon-coated tin, tin-coated carbon,carbon doped with tin, and tin doped with carbon). Examples of positiveelectrochemically active materials include various lithium metal oxides(e.g., LiCoO₂, LiFePO₄, LiMnO₂, LiNiO₂, LiMn₂O₄, LiCoPO₄,LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂, LiNi_(x)Co_(y)Al_(z)O₂, LiFe₂(SO₄)₃,Li₂FeSiO₄, Na₂FeO₄), carbon fluoride, metal fluorides such as ironfluoride (FeF₃), metal oxide, sulfur, and combinations thereof. Dopedand non-stoichiometric variations of these positive and negative activematerials may be used as well. Examples of dopants include elements fromthe groups III and V of the periodic table (e.g., boron, aluminum,gallium, indium, thallium, phosphorous, arsenic, antimony, and bismuth)as well as other appropriate dopants (e.g., sulfur and selenium). Incertain embodiments, a high capacity active material includes amorphoussilicon. For example, a layer of amorphous silicon may be deposited overnickel a silicide template.

High capacity active materials may be doped during or after thedeposition operation. Dopants can be used to improve conductivity of theactive material and to perform other functions. For example, phosphine(PH₃) may be added to the process gas to provide phosphorous doping ofsilicon or other active materials. In specific embodiments, such as someembodiments employing silane in the process gas, the concentration ofphosphine or another dopant carrying component in the process gas may beat least about at least about 0.1% (based on its partial pressure), orat least about 0.5%, or even at least about 1%. Dopants can be alsointroduced into the active layer after deposition of the active material(e.g., by sputtering, electroplating, ion implantation, and othertechniques). In certain embodiments, a lithium containing compound isdeposited onto the active material. The additional lithium may be usedin a lithium ion cell to offset losses associated with solid electrolyteinterface (SEI) layer formation and/or to keep some remaining lithiumpresent in the negative active material even during a complete celldischarge. Retaining some lithium in the negative electrode may help toimprove the negative active material conductivity and/or avoid certainmorphological changes in the negative active material at the end of thedischarge portion of the cycle.

In certain embodiments, multiple different active materials (e.g., highcapacity active materials such as tin) may be deposited over thetemplate. In one example, a layer of silicon may be further coated witha carbon layer to form a core-shell structure. In this example, thesilicide nanostructure of the template serves as a core, the siliconlayer as an intermediate layer or outer core, and the carbon layer as ashell. Other examples include coatings that include materials that arenot necessarily electrochemically active materials but that areconfigured to perform other functions in the electrode, such aspromoting the formation of a stable SEI layer. Examples of suchmaterials include carbon, copper, polymers, sulfides, and metal oxides.

In specific embodiments, an active material layer is deposited as acombination of germanium and silicon. The distribution of these twomaterials varies along the height of the template, such that moregermanium is deposited near the substrate interface than near the freeends, and vice versa for silicon. Germanium lithiates much less thansilicon and, as a result, germanium exhibits much less swelling. At thesame time, a morphological structure of germanium (e.g., its lattice)matches well to that of silicon. Lower swelling, in turn, helps toprotect the interface between the substrate and silicide structures,thereby resulting in more robust electrode structures and cells withimproved cycling performance.

The CVD process to form a variable composition active material layer maystart with introducing a process gas containing an initial concentrationof the germanium containing precursor and an initial concentration ofthe silicon containing precursor. The concentration of the germaniumcontaining precursor is then decreased, while the concentration of thesilicon containing precursor is increased.

High capacity active materials may be deposited using CVD techniques,electroplating, electroless plating, or solution deposition. In someembodiments, they are deposited in a manner similar to that employed togrow the silicide structures. Both silicides and active materials may bedeposited in the same chamber. More specifically, the same chamber maybe also used for the substrate treatment.

In certain embodiments, active materials may be deposited using a plasmaenhanced chemical vapor deposition (PECVD) technique. This techniquewill now be described in more detail with reference to an amorphoussilicon layer doped with phosphorous. However, it should be understoodthat this or similar techniques may be used for other active materialsas well. A substrate containing a silicide template, more specifically anickel silicide template, is provided in a PECVD chamber. The substrateis heated to between about 200° C. and 400° C. or, more specifically,between about 250° C. and 350° C. A process gas containing a siliconcontaining precursor (e.g., silane) and one or more carrier gases (e.g.,argon, nitrogen, helium, hydrogen, oxygen, carbon dioxide, and methane)is introduced into the chamber. In a specific example, a concentrationof silane in helium is between about 5% and 20% or, more specifically,between about 8% and 15%. The process gas may also include a dopantcontaining material, such as phosphine, at a concentration of betweenabout 1% and 5%. The chamber pressure may be maintained at between about0.1 Torr to 10 Torr or, more specifically, at between about 0.5 Torr and2 Torr. To enhance silane decomposition, a plasma is ignited in thechamber.

The following process (i.e., Radio Frequency (RF) power and flow rates)parameters are provided for an STS MESC Multiplex CVD system availablefrom Surface Technology Systems in United Kingdom, which can processsubstrates up to about 4 inches in diameter. It should be understood byone having ordinary skills in the art that these process parameters canbe scaled up or down for other types of chambers and substrate sizes.The RF power may be maintained at between about 10W and 100 W and theoverall process gas flow rate may be kept at between about 200 sccm and1000 sccm or, more specifically, at between about 400 sccm and 700 sccm.

In a specific embodiment, forming a layer of the electrochemicallyactive material is performed in a process chamber maintained at apressure of about 1 Torr. The process gas contains about 50 sccm ofsilane and about 500 sccm of helium. In order to dope the activematerial, about 50 sccm of 15% phosphine may be added to the processgas. The substrate is kept at about 300° C. The RF power level is set toabout 50 W. In certain embodiments, a pulsed PECVD method is employed.

To achieve an adequate thickness of the active material, deposition maybe performed for between about 0.5 minutes and 30 minutes. A thicknessof the active material may be driven by energy density requirements,material properties (e.g., theoretical capacity, stress fracturelimits), template surface area, and other parameters. In certainembodiments, a layer of amorphous silicon that is between about 50nanometers and 500 nanometers thick or, more specifically, between about100 nanometers and 300 nanometers thick, is deposited. It should benoted that this layer is deposited on silicide nanowires havingdiameters of between about 10 nanometers and 100 nanometers. Therefore,an average diameter of the resulting structure (i.e., silicide nanowireswith an active material layer deposited over the nanowires) may bebetween about 100 nanometers and 1,100 nanometers. Other dimensions maybe possible as well. For example, an amorphous silicon layer thickerthan about 500 nanometers is possible by increasing porosity of thelayer. In certain embodiments, a porous silicon layer may be betweenabout 500 nanometers and 1000 nanometers thick or, more specifically,between about 500 nanometers and 750 nanometers thick. Some examples anddetails of porous active material structures are provided in U.S.Provisional Patent Application 61/406,049, entitled “COMPOSITESTRUCTURES CONTAINING HIGH CAPACITY POROUS ACTIVE MATERIALS CONSTRAINEDIN SHELLS” filed on Oct. 22, 2010, which is incorporated herein byreference in its entirety for purposes of describing porous activematerial structures.

It has been determined that some active material layers havingthicknesses of between about 50 nanometers and 500 nanometers can betypically deposited within 10-20 minutes. Another way to characterize anamount of the deposited active material is relative to the underlyingtemplate. In certain embodiments, a mass ratio of the active materialvolume to the metal silicide volume is at least about 10 or, morespecifically, at least about 100. As described in other parts of thisdocument, this ratio may vary significantly along the height of thetemplate. Specifically, this ratio may be substantially less near thesubstrate interface than near the free ends of the individualstructures.

FIG. 3A illustrates four examples of the structures that are producedduring different stages of the overall process explained above. Asubstrate 302 may be initially provided during an initial stage 301. Asexplained above, a substrate 302 may include a base material and a metalsource material (used to form silicide). Various examples andcombinations of these materials are described above. The substrate 302may be then treated to form a surface 304 that is suitable to formsilicide nanostructures (stage 303). If the substrate 302 is a foil,surface 304 may be formed on both sides of the foil (not shown). In someexamples, surface 304 includes specific nucleation sites for formingnanowires. Surface 304 may also include masking materials. Silicidenanostructures 306 are then formed on the substrate 302 (stage 305). Incertain embodiments, silicide nanostructures 306 have their ends rootedto the substrate 302. Silicide nanostructures form a high surface areatemplate that is used for depositing an active material. Finally, anactive material layer 308 is deposited over the silicide nanostructures306 (stage 307). Silicide nanostructures 306 can provide both mechanicalsupport to the active material 308 and electrical connection to thesubstrate 302. While some contact may exist between the active materialand the substrate, it may not be sufficient from a battery performanceperspective.

A combination of the silicide nanostructures 306 and the active material308 may be referred to as an active layer 309, which is adjacent tosubstrate 302. Overall, active layer 309 may be characterized by itsheight, which is typically close to the height of the silicide templateor the length of the nanowires making this template. In certainembodiments, a height of the active layer is between about 10micrometers and 50 micrometers or, more specifically, between about 20micrometers and 40 micrometers. An electrode having a substrate and twoactive layers deposited on the two opposite sides of the substrate mayhave a height of between about 50 micrometers and 100 micrometers.Furthermore, active layer 309 may be characterized by its porosity(e.g., at least about 25% or, more specifically, at least about 50% or,even more specifically, at least about 75%), its capacity per unit area,and other characteristics.

Further, an amount of the active material coating the template may varyalong the height of the template. For example, an active material layermay be thicker near the free ends of the structures than near thesubstrate interface. FIG. 3B illustrates an example of such an activematerial layer 310 deposited over template structures 306 arranged on asubstrate 302. Without being restricted to any particular theory, it isbelieved that such distribution of the active material can be achievedby certain process conditions resulting in a mass transport limitingregime. This regime results in a concentration gradient of the activematerial precursor species (e.g., silane) along the height of thetemplate and higher deposition rates near the free ends of thestructures than near the substrate interface. Such active materialdistribution may be beneficial from a electrochemical cyclingperspective because the substrate rooted ends of the structures willexperience less swelling and stress during lithiation, therebypreserving contact between the structures and the substrate.

Specifically, uneven distribution of the active material may be achievedby performing CVD deposition at relative high pressure levels inside thedeposition chamber. Without being restricted to any particular theory,it is believed that a shorter mean free path is achieved at higherpressure levels, which, in turn, leads to high faster deposition ratesand rapid consumption of the active material precursors near the freeends of the structures. This effectively creates a mass transportlimiting regime over the height of the template. For example, depositionmay be performed at between about 50 Torr and 760 Torr, morespecifically at between about 100 Torr and 600 Torr or, even morespecifically, between about 200 Torr and 600 Torr. In a particularexample, deposition is performed at about 600 Torr. Depositiontemperatures may be between about 400° C. and 600° C. or, morespecifically, between about 450° C. and 550° C. In a particular example,deposition is performed at about 500° C. These temperature ranges arepresented for a thermal CVD technique. If a PECVD technique is used fordeposition, the temperatures may be in the range of between about 200°C. and 450° C. Silane concentration in argon or hydrogen may rangebetween about 0.5% and 20% or, more specifically, between about 0.5% and10% or, even more specifically, between about 1% and 5%.

Another approach is to perform a deposition using a PECVD technique at alow temperature. PECVD creates localized radicals that have a shorterlifetime than thermally excited radicals. Therefore, the mean free pathis believed to be shorter and deposition becomes less conformal, whichprovides more deposition at the top of the template where the radicalconcentration is greater. Also, PECVD allows deposition at lowertemperatures, as was mentioned above. Lower temperatures help reduceside reactions with the substrate and the forming of an unwanted excessof silicides at the substrate interface that may become brittle. A PECVDdeposition may be performed at pressure levels of between about 1 Torrand 50 Torr, temperature ranges of between about 200° C. and 450° C.,and a concentration of silane of between about 1% and 20% in hydrogen,helium, nitrogen, argon, or various combinations thereof. Plasma insidethe chamber may be biased to provide more desirable distribution of thereactive species.

Furthermore, a remote plasma generator may be used to create activatedspecies from the active material precursors, such as ions and radicals.The activated species (e.g., ⁻²SiH₂) are more reactive that theirun-activated counterparts (e.g., SiH₄) and tend to be consumed faster atthe free ends of the structures, thereby effectively creating a masstransport limiting regime. Some examples of the remote plasma generatorsinclude ASTRON® i Type AX7670, ASTRON® e Type AX7680, ASTRON® ex TypeAX7685, ASTRON® hf-s Type AX7645, which are all available from MKSInstruments of Andover, Mass. The generator is typically aself-contained device generating ionized plasma using the suppliedactive material precursors. The generator also includes a high power RFgenerator for supplying energy to the electrons in the plasma. Thisenergy may then be transferred to the neutral active material precursormolecules (e.g., silane) causing the temperature of these molecules toraise to a 2000K level and resulting in thermal dissociation of themolecules. The generator may dissociate more than 90% of the suppliedprecursor molecules because of its high RF energy and special channelgeometry that causes the precursors to adsorb most of this energy. Thegenerator may be used by itself (e.g., together with a Thermal CVDchamber) or in a combination with a PECVD reactor, which may providefurther dissociation of the species (e.g., species that were recombinedin the deliver line and shower head).

FIG. 4A is an SEM image of the silicide nanowires as viewed from above.These nanowires were deposited directly on a hard rolled nickel foilavailable from Carl Schlenk AG Company in Roth, Germany. The foil wasfirst oxidized for 1 min at 300° C. in a process chamber containing airat a pressure of 50 Torr. The foil was then heated to 450° C. and aprocess gas containing 1% by volume of silane was introduced into thechamber for 10 minutes. Resulting silicide nanowires were about 10-50nanometers in diameter and about 1-30 micrometers in length. A densityof nanowires was between about 10-70%. As can be seen in the SEM image,the nanowires form a very high surface area template. These templateswere then coated with amorphous silicon and used to construct coincells.

FIG. 4B is an SEM image of the nanowires coated with amorphous silicon.The image was taken from the same direction as was FIG. 4A. The initialsilicide template used for depositing the silicon is the same as in FIG.4A. Amorphous silicon deposition was performed at 300° C. and 1 Torr for10 minutes. The process gas included 50 sccm of 100% silane, 500 sccm ofhelium, and 50 sccm of 15% by volume phosphine. The RF power was 50 W.The average diameter of the coated nanowires was estimated to be 271-280nanometers. The SEM images of both FIGS. 4A and 4B are provided at thesame magnification to illustrate the relative sizes of the uncoatedtemplate nanowires (in FIG. 4A) and the amorphous silicon structureformed over these nanowires (in FIG. 4B). As can be seen from the twoSEM images, the amorphous silicon structures are substantially thickerthan the uncoated silicide nanowires.

FIG. 4C is a side view SEM image of the active layer containing siliconcoated nanowires similar to the ones in FIG. 4A. The nanowires have arelatively high aspect ratio even after being coated with the activematerial. The height of the active layer is generally defined by lengthof the nanowires. Further, an active layer has a relatively highporosity, which allows the nanowires to swell during lithiation withoutgenerating excessive stresses in the active layer and breaking eachother. The porosity also allows electrolyte components to freely migratethrough the active layer.

FIG. 4D illustrates a higher magnification SEM image of the active layeroriginally presented in FIG. 4B. Black arrows point to contact points(sometimes referred to herein as “interconnections”) between thenanowires. Such interconnections could have formed during deposition ofthe nickel silicide nanowires and/or coating the nanowires withamorphous silicon. As indicated above, such interconnections enhancemechanical strength and electrical conductivity of the active layer.

FIG. 4E is an SEM image obtained at an angle with respect to the topsurface of the electrode and illustrating nanowires being much thickerat their free ends than at their substrate-rooted ends. The activematerial structures forming this electrode have much thicker free endsthan substrate interface ends. Such structures are schematicallyillustrated in FIG. 3B and described above. It has been estimated thatthe structures shown in FIG. 4E have free ends that are about 1micrometer in diameter, while the substrate rooted ends are about 200nanometers in diameter. The length of the structures was estimated to beabout 12-20 micrometers.

Electrodes are typically assembled into a stack or a jelly roll. FIGS.5A and 5B illustrates a side and top views of an aligned stack includinga positive electrode 502, a negative electrode 504, and two sheets ofthe separator 506 a and 506 b, in accordance with certain embodiments.The positive electrode 502 may have a positive active layer 502 a and apositive uncoated substrate portion 502 b. Similarly, the negativeelectrode 504 may have a negative active layer 504 a and a negativeuncoated substrate portion 504 b. In many embodiments, the exposed areaof the negative active layer 504 a is slightly larger that the exposedarea of the positive active layer 502 a to ensure that most or alllithium ions released from the positive active layer 502 a go into thenegative active layer 504 a. In one embodiment, the negative activelayer 504 a extends at least between about 0.25 and 5 mm beyond thepositive active layer 502 a in one or more directions (typically alldirections). In a more specific embodiment, the negative layer extendsbeyond the positive layer by between about 1 and 2 mm in one or moredirections. In certain embodiments, the edges of the separator sheets506 a and 506 b extend beyond the outer edges of at least the negativeactive layer 504 a to provide electronic insulation of the electrodefrom the other battery components. The positive uncoated substrateportion 502 b may be used for connecting to the positive terminal andmay extend beyond negative electrode 504 and/or the separator sheets 506a and 506 b. Likewise, the negative uncoated portion 504 b may be usedfor connecting to the negative terminal and may extend beyond positiveelectrode 502 and/or the separator sheets 506 a and 506 b.

The positive electrode 502 is shown with two positive active layers 512a and 512 b on opposite sides of the flat positive current collector 502b. Similarly, the negative electrode 504 is shown with two negativeactive layers 514 a and 514 b on opposite sides of the flat negativecurrent collector. Any gaps between the positive active layer 512 a, itscorresponding separator sheet 506 a, and the corresponding negativeactive layer 514 a are usually minimal to non-existent, especially afterthe first cycle of the cell. The electrodes and the separators areeither tightly wound together in a jelly roll or are positioned in astack that is then inserted into a tight case. The electrodes and theseparator tend to swell inside the case after the electrolyte isintroduced, and the first cycles remove any gaps or dry areas as lithiumions cycle the two electrodes and through the separator.

A wound design is a common arrangement. Long and narrow electrodes arewound together with two sheets of separator into a sub-assembly(sometimes referred to as a jellyroll), which is shaped and sizedaccording to the internal dimensions of a curved, often cylindrical,case. FIG. 6A shows a top view of a jelly roll comprising a positiveelectrode 606 and a negative electrode 604. The white spaces between theelectrodes represent the separator sheets. The jelly roll is insertedinto a case 602. In some embodiments, the jellyroll may have a mandrel608 inserted in the center that establishes an initial winding diameterand prevents the inner winds from occupying the center axial region. Themandrel 608 may be made of conductive material, and, in someembodiments, it may be a part of a cell terminal. FIG. 6B presents aperspective view of the jelly roll with a positive tab 612 and anegative tab 614 extending from the jelly roll. The tabs may be weldedto the uncoated portions of the electrode substrates.

The length and width of the electrodes depend on the overall dimensionsof the cell and the heights of the active layers and current collector.For example, a conventional 18650 cell with 18 mm diameter and 65 mmlength may have electrodes that are between about 300 and 1000 mm long.Shorter electrodes corresponding to low rate/higher capacityapplications are thicker and have fewer winds.

A cylindrical design may be desirable for some lithium ion cells becausethe electrodes swell during cycling and exert pressure on the casing. Around casing may be made sufficiently thin and still maintain sufficientpressure. Prismatic cells may be similarly wound, but their case maybend along the longer sides from the internal pressure. Moreover, thepressure may not be even within different parts of the cells, and thecorners of the prismatic cell may be left empty. Empty pockets may notbe desirable within the lithium ions cells because electrodes tend to beunevenly pushed into these pockets during electrode swelling. Moreover,the electrolyte may aggregate and leave dry areas between the electrodesin the pockets, which negatively affects the lithium ion transportbetween the electrodes. Nevertheless, for certain applications, such asthose dictated by rectangular form factors, prismatic cells areappropriate. In some embodiments, prismatic cells employ stacks ofrectangular electrodes and separator sheets to avoid some of thedifficulties encountered with wound prismatic cells.

FIG. 7 illustrates a top view of a wound prismatic jellyroll positionsin a case 702. The jelly roll comprises a positive electrode 704 and anegative electrode 706. The white space between the electrodes isrepresentative of the separator sheets. The jelly roll is inserted intoa rectangular prismatic case. Unlike the cylindrical jellyrolls shown inFIGS. 6A and 6B, the winding of the prismatic jellyroll starts with aflat extended section in the middle of the jelly roll. In oneembodiment, the jelly roll may include a mandrel (not shown) in themiddle of the jellyroll onto which the electrodes and separator arewound.

FIG. 8A illustrates a side view of a stacked cell 800 that includes aplurality of sets (801 a, 801 b, and 801 c) of alternating positive andnegative electrodes and a separator in between the electrodes. A stackedcell can be made to almost any shape, which is particularly suitable forprismatic cells. However, such a cell typically requires multiple setsof positive and negative electrodes and a more complicated alignment ofthe electrodes. The current collector tabs typically extend from eachelectrode and connect to an overall current collector leading to thecell terminal.

Once the electrodes are arranged as described above, the cell is filledwith electrolyte. The electrolyte in lithium ions cells may be liquid,solid, or gel. The lithium ion cells with the solid electrolyte arereferred to as a lithium polymer cells.

A typical liquid electrolyte comprises one or more solvents and one ormore salts, at least one of which includes lithium. During the firstcharge cycle (sometimes referred to as a formation cycle), the organicsolvent in the electrolyte can partially decompose on the negativeelectrode surface to form a SEI layer. The interphase is generallyelectrically insulating but ionically conductive, thereby allowinglithium ions to pass through. The interphase also prevents decompositionof the electrolyte in the later charging sub-cycles.

Some examples of non-aqueous solvents suitable for some lithium ioncells include the following: cyclic carbonates (e.g., ethylene carbonate(EC), propylene carbonate (PC), butylene carbonate (BC) andvinylethylene carbonate (VEC)), vinylene carbonate (VC), lactones (e.g.,gamma-butyrolactone (GBL), gamma-valerolactone (GVL) and alpha-angelicalactone (AGL)), linear carbonates (e.g., dimethyl carbonate (DMC),methyl ethyl carbonate (MEC), diethyl carbonate (DEC), methyl propylcarbonate (MPC), dipropyl carbonate (DPC), methyl butyl carbonate (NBC)and dibutyl carbonate (DBC)), ethers (e.g., tetrahydrofuran (THF),2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane (DME),1,2-diethoxyethane and 1,2-dibutoxyethane), nitrites (e.g., acetonitrileand adiponitrile) linear esters (e.g., methyl propionate, methylpivalate, butyl pivalate and octyl pivalate), amides (e.g., dimethylformamide), organic phosphates (e.g., trimethyl phosphate and trioctylphosphate), organic compounds containing an S=0 group (e.g., dimethylsulfone and divinyl sulfone), and combinations thereof.

Non-aqueous liquid solvents can be employed in combination. Examples ofthese combinations include combinations of cyclic carbonate-linearcarbonate, cyclic carbonate-lactone, cyclic carbonate-lactone-linearcarbonate, cyclic carbonate-linear carbonate-lactone, cycliccarbonate-linear carbonate-ether, and cyclic carbonate-linearcarbonate-linear ester. In one embodiment, a cyclic carbonate may becombined with a linear ester. Moreover, a cyclic carbonate may becombined with a lactone and a linear ester. In a specific embodiment,the ratio of a cyclic carbonate to a linear ester is between about 1:9to 10:0, preferably 2:8 to 7:3, by volume.

A salt for liquid electrolytes may include one or more of the following:LiPF₆, LiBF₄, LiClO₄ LiAsF₆, LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, LiCF₃SO₃,LiC(CF₃SO₂)₃, LiPF₄(CF₃)₂, LiPF₃(C₂F₅)₃, LiPF₃(CF₃)₃, LiPF₃(iso-C₃F₇)₃,LiPF₅(iso-C₃F₇), lithium salts having cyclic alkyl groups (e.g.,(CF₂)₂(SO₂)_(2x)Li and (CF₂)₃(SO₂)_(2x)Li), and combinations thereof.Common combinations include LiPF₆ and LiBF₄, LiPF₆ and LiN(CF₃SO₂)₂,LiBF₄ and LiN(CF₃SO₂)₂.

In one embodiment, the total concentration of salt in a liquidnonaqueous solvent (or combination of solvents) is at least about 0.3 M;in a more specific embodiment, the salt concentration is at least about0.7M. The upper concentration limit may be driven by a solubility limitor may be no greater than about 2.5 M; in a more specific embodiment, itmay be no more than about 1.5 M.

A solid electrolyte is typically used without the separator because itserves as the separator itself. It is electrically insulating, ionicallyconductive, and electrochemically stable. In the solid electrolyteconfiguration, a lithium containing salt, which could be the same as forthe liquid electrolyte cells described above, is employed but ratherthan being dissolved in an organic solvent, it is held in a solidpolymer composite. Examples of solid polymer electrolytes may beionically conductive polymers prepared from monomers containing atomshaving lone pairs of electrons available for the lithium ions ofelectrolyte salts to attach to and move between during conduction, suchas polyvinylidene fluoride (PVDF) or chloride or copolymer of theirderivatives, poly(chlorotrifluoroethylene),poly(ethylene-chlorotrifluoro-ethylene), or poly(fluorinatedethylene-propylene), polyethylene oxide (PEO) and oxymethylene linkedPEO, PEO-PPO-PEO crosslinked with trifunctional urethane,poly(bis(methoxy-ethoxy-ethoxide))-phosphazene (MEEP), triol-type PEOcrosslinked with difunctional urethane,poly((oligo)oxyethylene)methacrylate-co-alkali metal methacrylate,polyacrylonitrile (PAN), polymethylmethacrylate (PNMA),polymethylacrylonitrile (PMAN), polysiloxanes and their copolymers andderivatives, acrylate-based polymer, other similar solvent-freepolymers, combinations of the foregoing polymers either condensed orcross-linked to form a different polymer, and physical mixtures of anyof the foregoing polymers. Other less conductive polymers that may beused in combination with the above polymers to improve the strength ofthin laminates include: polyester (PET), polypropylene (PP),polyethylene napthalate (PEN), polyvinylidene fluoride (PVDF),polycarbonate (PC), polyphenylene sulfide (PPS), andpolytetrafluoroethylene (PTFE).

FIG. 9 illustrates a cross-section view of a wound cylindrical cell, inaccordance with one embodiment. A jelly roll comprises a spirally woundpositive electrode 902, a negative electrode 904, and two sheets of theseparator 906. The jelly roll is inserted into a cell case 916, and acap 918 and gasket 920 are used to seal the cell. It should be notedthat in certain embodiments a cell is not sealed until after subsequentoperations. In some cases, cap 918 or cell case 916 includes a safetydevice. For example, a safety vent or burst valve may be employed toopen if excessive pressure builds up in the battery. In certainembodiments, a one-way gas release valve is included to release oxygenthat has been released during activation of the positive material. Also,a positive thermal coefficient (PTC) device may be incorporated into theconductive pathway of cap 918 to reduce the damage that might result ifthe cell suffered a short circuit. The external surface of the cap 918may used as the positive terminal, while the external surface of thecell case 916 may serve as the negative terminal. In an alternativeembodiment, the polarity of the battery is reversed and the externalsurface of the cap 918 is used as the negative terminal, while theexternal surface of the cell case 916 serves as the positive terminal.Tabs 908 and 910 may be used to establish a connection between thepositive and negative electrodes and the corresponding terminals.Appropriate insulating gaskets 914 and 912 may be inserted to preventthe possibility of internal shorting. For example, a Kapton™ film may beused for internal insulation. During fabrication, the cap 918 may becrimped to the cell case 916 in order to seal the cell. However, priorto this operation, electrolyte (not shown) is added to fill the porousspaces of the jelly roll.

A rigid case is typically used for lithium ion cells, while lithiumpolymer cells may be packed into flexible, foil-type (polymer laminate)cases. A variety of materials can be chosen for the cases. Forlithium-ion batteries, Ti-6-4, other Ti alloys, Al, Al alloys, and 300series stainless steels may be suitable for the positive conductive caseportions and end caps, and commercially pure Ti, Ti alloys, Cu, Al, Alalloys, Ni, Pb, and stainless steels may be suitable for the negativeconductive case portions and end caps.

In addition to the battery applications described above, metal silicidesmay be used in fuel cells (e.g., for anodes, cathodes, andelectrolytes), hetero junction solar cell active materials, variousforms of current collectors, and/or absorption coatings. Some of theseapplications can benefit from a high surface area provided by metalsilicide structures, high conductivity of silicide materials, and fastinexpensive deposition techniques.

1. An electrochemically active electrode material for use in a lithiumion cell, the electrochemically active electrode material comprising: ananostructured template comprising a metal silicide, the nanostructuredtemplate comprising nanowires rooted to a substrate, the nanowirescomprising substrate-rooted ends and free-ends; and non-silicide,electrochemically active material shells coating the nanowires, thenon-silicide electrochemically active material having a theoreticallithiation capacity of at least about 500 mAh/g.
 2. Theelectrochemically active electrode material of claim 1, wherein themetal silicide is selected from a group consisting of nickel silicide,cobalt silicide, copper silicide, silver silicide, chromium silicide,titanium silicide, aluminum silicide, zinc silicide, and iron silicide.3. The electrochemically active electrode material of claim 1, whereinthe electrochemically active material is selected from the groupconsisting of crystalline silicon, amorphous silicon, silicon oxides,silicon oxy-nitrides, tin-containing material, and germanium-containingmaterial.
 4. The electrochemically active electrode material of claim 1,wherein the shells are disjoined at the substrate.
 5. Theelectrochemically active electrode material of claim 1, wherein theshells are at least twice as thick at the free-ends of the nanowires asthey are at the substrate-rooted ends.
 6. The electrochemically activeelectrode material of claim 5, wherein the thickness of the shellsincreases gradually and continuously from the substrate rooted ends tothe free ends of the nanowires.
 7. The electrochemically activeelectrode material of claim 5, wherein the thickness of the shellsincreases gradually, but not continuously, from the substrate rootedends to the free ends of the nanowires.
 8. The electrochemically activeelectrode material of claim 5, wherein the thickness of the shellsincreases abruptly and continuously from the substrate rooted ends tothe free ends of the nanowires.
 9. The electrochemically activeelectrode material of claim 5, wherein the thickness of the shellsincreases abruptly, but not continuously, from the substrate rooted endsto the free ends of the nanowires.
 10. The electrochemically activeelectrode material of claim 9, wherein the shells comprise poroussilicon.
 11. The electrochemically active electrode material of claim 1,wherein the nanowires are uncoated with electrochemically activematerial at or near their interfaces with the substrate.
 12. A lithiumion electrode for use in a lithium ion cell, the lithium ion electrodecomprising: a current collector substrate; a nanostructured templatecomprising a metal silicide, the nanostructured template comprisingnanowires rooted to the current collector substrate, the nanowirescomprising substrate-rooted ends and free-ends; and non-silicide,electrochemically active material shells coating the nanowires, thenon-silicide electrochemically active material having a theoreticallithiation capacity of at least about 500 mAh/g.
 13. A lithium ion cellcomprising: a current collector substrate; a nanostructured templatecomprising a metal silicide, the nanostructured template comprisingnanowires rooted to the current collector substrate, the nanowirescomprising substrate-rooted ends and free-ends; and non-silicide,electrochemically active material shells coating the nanowires, thenon-silicide electrochemically active material having a theoreticallithiation capacity of at least about 500 mAh/g.
 14. Theelectrochemically active electrode material of claim 1, furthercomprising a dopant in the non-silicide electrochemically activematerial.
 15. The electrochemically active electrode material of claim14, wherein the dopant is selected from the group consisting of boron,aluminum, gallium, indium, thallium, phosphorous, arsenic, antimony,bismuth, sulfur, and selenium.